Accurate dispensing of needed volumes of chemicals is critical to manufacturing processes in a variety of industries. For example, in semiconductor fabrication processes, wafers are immersed in treatment tanks containing process chemicals. Some of the processing steps required for semiconductor processing are highly concentration dependent, because variations in chemical concentrations can result in variations in surface properties between different batches of wafers. It is thus essential to precisely regulate the concentration of the chemicals dispensed into the treatment tanks so as to insure that the finished wafers have the desired surface properties and to further insure consistent processing between wafer batches.
This need is oftentimes at odds with typical operation of the bulk chemical supplies utilized within fabrication facilities to supply chemical for wafer processing. For example, the bulk supplies have flow pressures that vary greatly over time. Time-based dispensing, in which a desired volume is dispensed from the bulk supply simply by regulating the time for which the chemical is allowed to flow into the process tank, is inherently inaccurate in this situation. Without a substantially constant flow rate, the dispense "on" time for the needed volume cannot be accurately calculated. This leads to inconsistent dispensing between successive batches of wafers. As another example, bulk supplies can often shut down for periods of time, leaving no chemical available for dispensing into the treatment tanks. Each of these difficulties can adversely affect the profitability of the fabrication facilities by yielding products having inferior or inconsistent surface characteristics or by forcing temporary shutdown of processing steps. It is therefore desirable to provide a dispensing system and method that will accurately and consistently dispense chemicals regardless of the condition of the bulk source supplying the chemicals.
One solution to the problem of bulk flow rate variations is to precisely measure a volume of chemical being dispensed from the bulk supply using a fluid level sensor positioned in the process tank. The fluid sensor provides feedback that triggers termination of chemical flow into the vessel once the desired volume has been dispensed.
While this solution increases accuracy in dispensing, it fails to address the problem of bulk supply shut down and is impractical for use with process tanks for which the required chemical volume may vary. Multiple fluid level sensors would be required to give the system sufficient versatility to provide varying amounts of chemical, and even then its use would be limited to a finite number of available quantities. Moreover, the need for accurate dispensing requires a slow fluid flow rate into the process vessel, so as to allow sufficient time for flow to be stopped before the dispensed volume significantly exceeds the level detected by the fluid sensor. This mandates a lengthy fill time prior to each use of the treatment tank and thus detracts from the number of wafer batches that can be treated per day.
It is thus desirable to provide a system and method for accurately dispensing chemicals from a bulk supply while minimizing reliance on the flow pressures of the bulk supply. It is further desirable to provide a system that, in addition to having these attributes, is easily adaptable to demands for varying concentrations, which permits quick dispensing of chemicals into process vessels so as to avoid delays between process steps, and which allows processing to continue through bulk supply down times.